multiboot - Multiboot 2 structure definitions

Header: kernel/include/kernel/multiboot.h

Type definitions and constants for the Multiboot 2 boot protocol. Makar is loaded by GRUB, which places a pointer to the Multiboot 2 information structure in the ebx register before transferring control to _start.

Magic value

#define MULTIBOOT2_BOOTLOADER_MAGIC  0x36D76289

The bootloader places this value in eax at entry. kernel_main passes it to pmm_init(), which validates it before trusting the memory map. The GDB boot test also verifies this value at _start.

Tag type constants

Constant Value Description
MULTIBOOT2_TAG_TYPE_END 0 Terminates the tag list.
MULTIBOOT2_TAG_TYPE_MMAP 6 Physical memory map.
MULTIBOOT2_TAG_TYPE_FRAMEBUFFER 8 Linear framebuffer info.

Memory type constants

Constant Value Description
MULTIBOOT2_MEMORY_AVAILABLE 1 RAM usable by the OS.

Framebuffer type constants

Constant Value Description
MULTIBOOT2_FRAMEBUFFER_TYPE_INDEXED 0 Palette/indexed colour.
MULTIBOOT2_FRAMEBUFFER_TYPE_RGB 1 Direct/packed RGB colour.
MULTIBOOT2_FRAMEBUFFER_TYPE_EGA_TEXT 2 EGA text mode.

Structures

multiboot2_info_t

typedef struct {
    uint32_t total_size;
    uint32_t reserved;
} __attribute__((packed)) multiboot2_info_t;

The 8-byte header at the very start of the information structure. total_size includes the header itself. Tags begin at (uint8_t *)mbi + sizeof(multiboot2_info_t).

multiboot2_tag_t

typedef struct {
    uint32_t type;
    uint32_t size;
} __attribute__((packed)) multiboot2_tag_t;

Generic tag header. All tags begin with these two fields. To advance to the next tag: ptr += (tag->size + 7) & ~7 (tags are 8-byte aligned).

multiboot2_mmap_entry_t

typedef struct {
    uint64_t base_addr;
    uint64_t length;
    uint32_t type;
    uint32_t reserved;
} __attribute__((packed)) multiboot2_mmap_entry_t;

One entry in the physical memory map. type == MULTIBOOT2_MEMORY_AVAILABLE identifies usable RAM.

multiboot2_tag_mmap_t

typedef struct {
    uint32_t type;
    uint32_t size;
    uint32_t entry_size;
    uint32_t entry_version;
    /* multiboot2_mmap_entry_t entries[] follow */
} __attribute__((packed)) multiboot2_tag_mmap_t;

Memory map tag (type 6). Individual entries begin at (uint8_t *)tag + sizeof(multiboot2_tag_mmap_t) and are each entry_size bytes apart.

multiboot2_tag_framebuffer_t

typedef struct {
    uint32_t type;
    uint32_t size;
    uint64_t framebuffer_addr;
    uint32_t framebuffer_pitch;
    uint32_t framebuffer_width;
    uint32_t framebuffer_height;
    uint8_t  framebuffer_bpp;
    uint8_t  framebuffer_type;
    /* RGB channel layout fields follow for type == RGB */
    uint8_t  red_field_position;
    uint8_t  red_mask_size;
    uint8_t  green_field_position;
    uint8_t  green_mask_size;
    uint8_t  blue_field_position;
    uint8_t  blue_mask_size;
} __attribute__((packed)) multiboot2_tag_framebuffer_t;

Framebuffer tag (type 8). framebuffer_addr is the physical base address of the linear framebuffer. The RGB shift and mask fields describe how to compose a pixel value for the given bpp.